Polyethyleneimine (PEI) is an important polymer that is useful in a wide range of commercial, biomedical, and research applications. PEI has been used as an excipient for pharmaceutical formulations, as a gene transfection agent, as an interface modifier for organic electronics, as a component in ion exchange resins, as a dye uptake modifier in the textile industry, as a gas absorber, and as a metal ion chelation agent for waste water treatment. It has also been used during the deposition of metals onto surfaces.
Certain forms of PEI can be formed by the cationic ring-opening polymerization (CROP) of aziridine. However, this is a non-selective process, giving a heterogeneous mixture of highly branched polymers:

The high degree of branching is the consequence of secondary amines along an existing polymer chain undergoing ring-opening reaction with unreacted aziridine monomers. The CROP of aziridine to form branched PEI is difficult to control and reproducibility between reaction batches is problematic. Due to the uncontrolled nature of the cationic polymerization heterogeneously branched polyethyleneimine have not been widely adopted in fields requiring batch uniformity, e.g., biomedical sciences and advanced manufacturing.
Linear PEI is known, and does not suffer from the heterogeneity problems associated with branched PEI. However, it has been more difficult to obtain linear PEI. It is not possible to synthesize linear unsubstituted PEI by a controlled polymerization directly from unsubstituted aziridine. Although installation of an electron withdrawing group (such as methanesulfonyl) at the nitrogen atom of an unsubstituted aziridine permits reaction via an anionic pathway to take place, it is not easy to obtain high molecular weight polymers by this method. The initially formed ethyleneimine oligomers are substantially insoluble in most solvents, and as they are formed they crystallize/precipitate from solution, effectively terminating the polymerization process.
2-alkyl-N-(methylsulfonyl)aziridine has been polymerized under living anionic conditions. Post-polymerization, the sulfonyl groups can be removed using lithium naphthalide to yield the linear PEI.

There are significant limitations to this technology. In particular, the polymerization proceeds to high molecular weight only if the N-sulfonylaziridine monomer is 2-substituted with an alkyl group and only if the monomer is racemic. If an enantiopure aziridine monomer is used, the growing polymer chain precipitates at low molecular weight. Furthermore, because the aziridine monomer must be substituted with an alkyl group for the polymerization to proceed, the final polymer has a lower nitrogen-to-carbon ratio than a polymer made from unsubstituted aziridine monomers.
As an alternative to polymerization of aziridine monomers, the polymerization of 2-oxazolines to form polyoxazolines (PDXs) has been explored. The resulting amide can be hydrolyzed to produce linear PEI.

Although this method can be used to obtain high molecular weight polymers, they typically have broader molecular weight distributions due to chain-transfer and chain-coupling reactions. Furthermore, the necessary post-polymerization hydrolysis does not easily go to completion, even under harsh reaction conditions. As such, the resulting polyethyleneimine product can be contaminated with amide bearing polymers.
What are needed are controllable and robust methods for producing linear polyethyleneimine, especially unsubstituted polyethyleneimine. Also needed are linear polyethyleneimines with narrow molecular weight distributions, and linear polyethyleneimines that are not contaminated with partially amidated polyethyleneimine. The methods and compositions disclosed herein addresses these and other needs.